Spectators try to stay cool with fans and umbrellas while waiting for play to start at the 2014 U.S. Open tennis tournament, Tuesday, Sept. 2, 2014, in New York.(John Minchillo/AP)

Venus Williams of the U.S. and her sister Serena play Ekaterina Makarova and Elena Vesnina of Russia in their quarter-final doubles match at the 2014 U.S. Open tennis tournament in New York, September 2, 2014.(RAY STUBBLEBINE/REUTERS)

Elena Vesnina, left, and Ekaterina Makarova of Russia celebrate after defeating Venus and Serena Williams of the U.S. in their quarter-final doubles match at the 2014 U.S. Open tennis tournament in New York, September 2, 2014.(RAY STUBBLEBINE/REUTERS)

Belinda Bencic, of Switzerland, returns a shot against Peng Shuai, of China, during the quarter-finals of the 2014 U.S. Open tennis tournament, Tuesday, Sept. 2, 2014, in New York.(Mike Groll/AP)

Peng Shuai, of China, returns a shot against Belinda Bencic, of Switzerland, during the quarter-finals of the 2014 U.S. Open tennis tournament, Tuesday, Sept. 2, 2014, in New York.(Mike Groll/AP)

Belinda Bencic of Switzerland serves to Peng Shuai of China during their quarter-finals match at the 2014 U.S. Open tennis tournament in New York, September 2, 2014.(ADAM HUNGER/REUTERS)

Peng Shuai serves during her match against Belinda Bencic on day nine of the 2014 U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center.(Jerry Lai/USA Today Sports)

Belinda Bencic of Switzerland misses a return to Peng Shuai of China during their quarter-finals match at the 2014 U.S. Open tennis tournament in New York, September 2, 2014.(ADAM HUNGER/REUTERS)

Peng Shuai of China hits a return to Belinda Bencic of Switzerland during their quarter-finals match at the 2014 U.S. Open tennis tournament in New York, September 2, 2014.(ADAM HUNGER/REUTERS)

Gael Monfils, of France, returns a shot to Grigor Dimitrov, of Bulgaria, during the fourth round of the 2014 U.S. Open tennis tournament, Tuesday, Sept. 2, 2014, in New York.(Mike Groll/AP)

Grigor Dimitrov, of Bulgaria, wipes sweat from his brow between points against Gael Monfils, of France, during the fourth round of the 2014 U.S. Open tennis tournament, Tuesday, Sept. 2, 2014, in New York.(Mike Groll/AP)

Grigor Dimitrov of Bulgaria hits a return to Gael Monfils of France during their fourth round match at the 2014 U.S. Open tennis tournament in New York, September 2, 2014.(ADAM HUNGER/REUTERS)

Spectators watch fourth round play at Louis Armstrong Stadium during the 2014 U.S. Open tennis tournament, Tuesday, Sept. 2, 2014, in New York.(John Minchillo/AP)

Gael Monfils, of France, reacts after a shot against Grigor Dimitrov, of Bulgaria, during the fourth round of the 2014 U.S. Open tennis tournament, Tuesday, Sept. 2, 2014, in New York.(Mike Groll/AP)

Gilles Simon returns a shot against Marin Cilic on day nine of the 2014 U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center.(Jerry Lai/USA Today Sports)

Gael Monfils of France hits a return to Grigor Dimitrov of Bulgaria during their fourth round match at the 2014 U.S. Open tennis tournament in New York, September 2, 2014.(ADAM HUNGER/REUTERS)

Grigor Dimitrov, of Bulgaria, runs off the court to retrieve a new pair of shoes between games against Gael Monfils, of France, during the fourth round of the 2014 U.S. Open tennis tournament, Tuesday, Sept. 2, 2014, in New York.(Mike Groll/AP)

Gilles Simon, of France, serves against Marin Cilic of Croatia, during the fourth round of the 2014 U.S. Open tennis tournament, Tuesday, Sept. 2, 2014, in New York.(John Minchillo/AP)

Gael Monfils, of France, douses his face with water between games against Grigor Dimitrov, of Bulgaria, during the fourth round of the 2014 U.S. Open tennis tournament, Tuesday, Sept. 2, 2014, in New York.(Mike Groll/AP)